In a WDM optical transmission system that amplifies and repeatedly transmits WDM light with an optical repeater arranged on a transmission path, the intensity of the WDM light amplified by the optical repeater to be input to a transmission path fiber that connects to an output side of the optical repeater (hereunder, referred to as fiber input light intensity) is one of the important parameters that determine the characteristic of the system. If this fiber input light intensity becomes too low, the optical signal to noise ratio (OSNR) becomes deteriorated. On the other hand, if the fiber input light intensity becomes too high, the transmission quality becomes deteriorated due to the nonlinear effect within the optical fiber. Consequently, generally in an optical repeater, the intensity of the WDM light amplified with use of an optical amplifier is monitored, and according to the monitoring results, the optical amplifier is controlled so that the light intensity per one channel of the WDM light becomes constant. This channel means an optical signal of each wavelength contained in the WDM light.
However, for example as illustrated at the top of FIG. 14, in a system in which a plurality of optical repeaters 104 are arranged on a transmission path 103 between a transmitting station 101 and a receiving station 102, and WDM light is sequentially amplified and transmitted by each optical repeater 104, optical noise that occurs in an optical amplifier within each optical repeater accumulates. Consequently, even if a control in each optical repeater 104 is performed to make the light intensity per one channel constant, as illustrated on the bottom of FIG. 14, the proportion of signal components among signal components and noise components contained in each channel gradually decreases with an increase in the number of repeaters.
FIG. 15 is an example illustrating changes in the proportion of the light intensity of the signal component and the light intensity of the noise component according to the number of spans, for the WDM light input to the transmission path of each repeater section (span). Moreover, FIG. 16 is an example illustrating changes in the signal light intensity in one channel according to the number of spans. As seen in the example of each diagram, each channel of the WDM light is such that even if the light intensity for where the signal component and the noise component are combined (hereunder, referred to as total light intensity) is controlled to be constant, the noise light intensity increases with an increase in the number of spans, and the signal light intensity decreases. Therefore, the actual signal light intensity (square dots in FIG. 16) becomes smaller than the required signal light intensity (rhombic dots in FIG. 16) at which reception processing of each channel can be normally performed under the control to make the total light intensity constant, and the difference thereof increases corresponding to the increase in the number of spans.
As a conventional technique to reduce such OSNR deterioration, for example as illustrated in FIG. 17, for the control of an optical amplifier in each optical repeater, there has been proposed a method of correcting a control target value of the total light intensity per one channel, to correspond to the increase in the noise light intensity due to the increase in the number of spans (for example, refer to International Publication Pamphlet No. WO 02/021203). According to this conventional technique, as illustrated in FIG. 18, the signal light intensity is maintained at a constant level even if the noise light intensity increases due to the increase in the number of spans. Therefore a required signal light intensity can be achieved regardless of the number of spans.
To describe specifically, for example, when a WDM optical transmission system illustrated at the top of FIG. 19 in which a plurality of OADM nodes 105 are arranged on the transmission path 103, in each OADM node 105, the WDM light transmitted through the transmission path 103 is amplified to a required level by an input side optical amplifier, and is then provided to an OADM unit. In the OADM unit, a demultiplexer demultiplexes the WDM light for each channel, an add/drop processing is performed for each channel, and the respective channels are multiplexed again by a multiplexer. Then the WDM light processed in the OADM unit is amplified to a required level by an output side optical amplifier, and sent to the transmission path 103.
In such an OADM node 105, when the WDM light is demultiplexed for each channel in the demultiplexer in the OADM unit, noise components distributed across the intermediate wavelength region of the respective channels are filtered and removed. Consequently, the noise light contained in the WDM light processed in the OADM node 105 only has remaining components corresponding to the wavelength width of each channel as illustrated at the bottom of FIG. 19. Therefore, the WDM light sent from the OADM node 105 to the transmission path 103, compared to the WDM light (refer to the bottom of FIG. 17) sent from an optical repeater node 104 that contains an optical amplifier but does not contain an OADM (hereunder, may be referred to as in-line amplifier (ILA)), to the transmission path 103, has its noise components reduced.
With respect to the above OADM node 105 that filters the noise component, in the case of performing, with the conventional technique mentioned above, a correction of the control target value of the total light intensity similar to that in the case of the optical repeater node 104, the amount of increase in the noise component is estimated at a level greater than that of the actual increase. Therefore the control target value of the total light intensity is excessively corrected. If a constant control of the output light intensity is performed in the OADM node 105, according to the control target value of the total light intensity corrected in this way, the signal light intensity of each channel of the WDM light input to the output side transmission path 103 of the OADM node 105 exceeds the required level thereof, and there is a possibility of a transmission quality deterioration due to the nonlinear effect.
FIG. 20 is an example illustrating changes in the proportion of the light intensity of the signal component and the light intensity of the noise component according to the number of spans, for the WDM light input to the transmission path between the respective repeater sections in the case where the OADM nodes are arranged in multiple stages on the transmission path. Moreover, FIG. 21 is an example illustrating changes in the signal light intensity in one channel according to the number of spans. As described above, each channel of the WDM light is such that the proportion of the signal light intensity with respect to the noise light intensity increases with the increase in the number of spans, the actual signal light intensity (square dots in FIG. 21) becomes greater than the required signal light intensity (rhombic dots in FIG. 21), and the difference therebetween expands corresponding to the increase in the number of the spans. Consequently, transmission quality deterioration due to the nonlinear effect is likely to occur in a system containing a number of OADM nodes on a transmission path thereof.